Provided is a method for treating wastewater, the method being capable, even during low water temperature periods, of efficiently purifying organic wastewater that experiences large temperature differences in a year. A method for treating organic wastewater uses a device including a predetermined reaction tank containing activated sludge and a culture tank and includes the steps of: culturing, in the culture tank, a microorganism contained in the activated sludge, while maintaining a temperature of a content of the culture tank at a temperature that is at least 5° C. lower than a water temperature of the organic wastewater at a start of culturing of the microorganism and not lower than −1° C. and not higher than 10° C.; and then introducing the microorganism into the reaction tank.